The present invention relates generally to methods and compositions for fracturing well formations, and more particularly, but not by way of limitation, to methods and compositions for fracturing well formations penetrated by a well bore wherein a fracturing fluid composition, with or without propping agent suspended therein, is injected into the formations through the tubing or casing at a rate sufficient to open a fracture in the formation.
Fracturing porous subterranean formations penetrated by a well bore has been widely employed for increasing the production of fluids, such as, crude oil, natural gas, and the like from said formations. The usual technique of fracturing a formation comprises introducing a fluid into the well under sufficient pressure to force the fluid out into the formation to fracture the formation and thereby alter the formation's permeability. The technique is not limited to formations of low permeability such as certain limestones, dolomite and the like, but also is applicable to other types of formations such as a sandstone containing streaks or striations of relatively high permeability and other zones of low permeability.
During the pressured injection of the gelled compositions described herein, passageways for fluid flow are created in the formation, or existing passageways therein are enlarged, thus stimulating the production of fluids from the formation.
Hydraulic fracturing is widely used to stimulate production from oil and gas wells completed in low permeability formations, and many methods and compositions for hydraulically fracturing subterranean well formations penetrated by a well bore have been developed.
Commonly, a fracturing fluid is pumped through the tubing or casing disposed in the well bore into the formation sought to be fractured. The fracturing fluid is pumped at a rate sufficient to open a fracture in the exposed formation, and extend the fracture from the well bore into the formation. Continued pumping of said fracturing fluid containing a propping agent into said fracture results in proppant placement within the fractured zone. Following the treatment, the fracturing fluid is recovered from the well, leaving the proppant remaining in the fracture, thereby preventing the complete closure thereof and forming a permeable channel extending from the well bore into the formation.
The conductivity of the propped fracture depends, among other things, on the size of the propping agent particles placed in the fracture. This, in turn, depends upon the width to which the particular fracture may be opened during injection of the fracturing fluid, and this normally requires that such fluids have very high viscosities. The use of fracturing fluids having relatively high viscosities is advantageous since such fluids can support the propping agent particles suspended therein without excessive settling. Also, relatively large size propping agent particles can be placed in the formation using high viscosity fracturing fluids since wider fractures generally result, reducing the possibility of the propping agent bridging across the mouth of the fracture and accumulating in the well bore, a condition commonly referred to as "screen-out".
A problem encountered in fracturing operations, particularly when employing compositions having thickening or viscosifying agents incorporated therein, is stability to heat. By stability to heat is meant the retention of the increased or greater viscosity properties under the conditions of use. Such compositions to be satisfactory should be sufficiently stable to resist degeneration by the heat of the formation for a period of time sufficient to accomplish the intended purpose, that is, good penetration and significant fracturing of the formation. The degree of stability required in any particular operation will vary with such operating variables as the type of formation being treated, the temperature of the formation, the well depth (time to pump the gelled composition down the well and into the formation), the polymer concentration of the composition, and the like.
The temperature of the formation usually has a pronounced effect on the stability of the gelled compositions and, generally speaking, is one of the most important operating variables when considering stability. Increased formation temperatures usually have at least one undesirable effect. Such an effect can be degeneration of the compositions, that is, a decrease in viscosity. Thus, some compositions which would be satisfactory in a low temperature formation might not be satisfactory in formations having higher temperatures, such as are encountered in deeper wells.
In certain fracturing operations using unthickened fluid there is usually no problem in removing the injected fluid because it is essentially water. However, a problem which is sometimes encountered when using thickened compositions in treating formations is the ease of removal of the treating composition after the operation is completed. Some thickened or highly viscous solutions are difficult to remove from the pores of the formation or the fracture after the operation is complete. Sometimes a clogging residue can be left in the pores of the formation or in the fracture. This can inhibit the production of fluids from the formation and can require costly cleanup operations. It would be desirable to have gelled compositions which break down to a lesser viscosity within a short time after the operation is completed.
Presently, guar gum and guar gum derivatives along with other natural gums are utilized in the preparation of viscous fluids utilized in the treatment of subterranean formations to provide adequate viscosity to the fluid when utilized at temperatures in excess of about 125.degree. F. to about 350.degree. F. Guar gum and the guar derivatives have the capability of being crosslinked by a variety of polyvalent metals, such as, for example, titanium, zirconium, chromium, antimony, niobium, and the like. While not entirely satisfactory, these viscosifiers form a "gelled" fracturing fluid having a sufficient viscosity to transport proppant into a subterranean formation. However, guar and the other gums typically contain substantial amounts, that is, from about 1.5 to in excess of 10 percent by weight of insoluble matter. The presence of such insoluble matter in a fluid is highly undesirable since it may clog the pores of the formation or the fracture.
It would appear that cellulose ethers, such as hydroxyethylcellulose, would be a viable alternative to guar gum, guar derivatives or other gums for use in fracturing fluids because of its substantially lower insoluble matter content. However, such has not been the case because the cellulose ethers have generally been unable to provide fracturing fluids having the necessary viscosity at elevated temperatures which can be achieved through the use of guar gum and guar derivatives. Further, hydroxyethylcellulose and its derivatives generally have not been capable of adequate crosslinking, which is prerequisite necessary, to provide sufficient viscosity levels for more severe applications.
The present invention provides a solution for, or at least mitigates, the above-discussed problems. The present invention provides improved methods for fracturing subterranean formations utilizing a novel gelling agent.